Thread: Two gas furnaces venting in same chimney

Two story home has been split into two different living spaces. One upstairs and one downstairs. 100,000 btu furnace in basement services the first floor and basement area and vents into the centrally located chimney(clay liner). 100,000 btu furnace in closet on second floor services that living area. It also vents into the same chimney. Both furnaces are 25+ years old. I called out the furnaces for venting into the same chimney when the units are not on the same floor. I was under the impression that two gas appliances can vent into the same chimney as long as they are on the same level. Can two furnaces vent into the same chimney and can they be on different levels of the home? HVAC tech says this set-up is okay. I am still waiting for a few HVAC techs I know to respond. My thanks in advance for any replies.

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Originally Posted by Jody Humbert

Can two furnaces vent into the same chimney and can they be on different levels of the home?

Yes they can.

I agree with Eric if the venting is properly sized and connected. How many FAUs and WHs have you seen tied into the same chimney flue?

Shouldn't they both be vented into a stainless steel liner? Aren't clay lined chimneys for solid fuel appliances?

It seems UMC 809.1 at least used to require all connectors entering on the same floor. Maybe some locales still do?

2012 IMC

801.19 Multistory prohibited.

Common venting systems for
appliances located on more than one floor level shall be prohibited,
except where all of the appliances served by the common
vent are located in rooms or spaces that are accessed
only from the outdoors. The appliance enclosures shall not

communicate with the occupiable areas of the building.

I have seen alot of gas water heaters and GFA units venting into the same chimney. They were all on the same level. I have seen multiple furnaces in one structure venting out through their own flues. I had never observed two furnaces on different floors venting into the same chimney. Thanks for the exact section of the IMC that mentions this, Fred. Local code authority is checking on whether this county follows this or not. He said he had never saw this before and didn't know the answer.

The original question was about venting into the same chimney.

If the question was intended to ask about venting into the same flue then my answer is different.

Eric makes the important distinction and why terminology is so important. You can have multiple flues in one chimney.

Just because you see a clay flue tile doesn't mean a chimney is "lined". Most chimneys I have ever inspected that used to be considered 'lined' had defects in them. Many we unlined with only one tile at the top to fool people into thinking it was lined or to make it look better or allow a std. cap to attach. That is a big reason for level II inspections.

Section 801.19 is in the General Section of Chapter 8 of the IMC, Chimneys and Vents. The answer is the same whether it is a chimney, a vent, or a flue. From the IMC definitions

CHIMNEY. A primarily vertical structure containing one or
more flues, for the purpose of carrying gaseous products of
combustion and air from a fuel-burning appliance to the outdoor
atmosphere.
Factory-built chimney. A listed and labeled chimney
composed of factory-made components, assembled in the
field in accordance with manufacturer’s instructions and
the conditions of the listing.
Masonry chimney. A field-constructed chimney composed
of solid masonry units, bricks, stones or concrete.
Metal chimney. A field-constructed chimney of metal.

FLUE. A passageway within a chimney or vent through
which gaseous combustion products pass.

VENT. A pipe or other conduit composed of factory-made
components, containing a passageway for conveying combustion
products and air to the atmosphere, listed and labeled for
use with a specific type or class of appliance.

My apologies on the wrong designation, Eric. It was the same flue in the same chimney. I have called it out as being wrong. HVAC techs I talked to didn't really know whether it was right or wrong. County inspector didn't return my phone call. They don't like to be bothered with anything that applies to houses older than 10 years. Thanks for the replies.

Originally Posted by Jody Humbert

HVAC techs I talked to didn't really know whether it was right or wrong.

I don't find that hard to believe.